1. Field of the Invention
The present invention relates to a method and an apparatus for X-ray or infrared imaging of a body, comprising in particular a support for receiving a body to be examined, a source emitting a beam of X-rays or light rays in a propagation direction in order to irradiate or illuminate the body to be examined, a detector irradiated or illuminated by the beam in order to detect an intensity attenuated according to the passage of the X-rays or light rays through the body to be examined, an analogue-digital converter to convert the detected intensities into data, enabling an attenuation of the X-rays or light rays by the body to be examined to be determined.
2. Description of the Related Art
Such an apparatus is known for example from U.S. Pat. Nos. 3,924,131 or 3,919,552. It will be recalled that scanography (or tomodensitometry) was discovered in 1968 by G. N. Hounsfield, an engineer working in the EMI company. The 1972 patent is entitled: “A method and apparatus for examination of a body by radiation such as X or gamma radiation”. In 1979 the inventor was awarded the Nobel prize for his invention.
The principle of the invention is as follows:
A beam of X-rays scans a defined plane, passes linearly through an organ, and strikes a plate or a radiographic detector. The passage through the organ produces an attenuation of the beam, the degree of attenuation being able to be measured by means of the detector. Crosswise scanning in the sectional plane produces a set of information that is processed by suitable software on an associated computer.
In fact, in a heterogeneous medium the attenuation along each scanning axis may be expressed by an exponential law, taking into account the photoelectric absorption and diffusion due to the Compton effect.
Let I0 be the reference value
Ix be the value at a point X,
then one may write the following relationship:
                              ∫                                    A              ⁡                              (                x                )                                      ⁢                          ⅆ              x                                      =        F                                l        =                  lo          ⁢                                          ⁢                      e                          -              F                                                              ln        =                  lo          ⁢                                          ⁢          E                                        Ln        =                              Io            Ik                    =                                    ∫              0              t                        ⁢                                          A                ⁡                                  (                  x                  )                                            ⁢                                                          ⁢                              ⅆ                x                                                        
From which one obtains by discretisation:
      Ln    ⁢          Io      In        =                    ∫        0        t            ⁢                        A          ⁡                      (            x            )                          ⁢                                  ⁢                  ⅆ          x                      =                            A          1                ⁢                  X          1                    +                        A          2                ⁢                  X          2                    +      …      +                        A          n                ⁢                  X          n                    
The successive values A1, A2 . . . , An correspond to the values of each segment defined by X1, X2 . . . , Xn.
The profiles of each scanning associated with a specific angle (or a specific position) may then be expressed by a series of equations.
A particular scale may be defined by the value relative to a reference value of the coefficient of attenuation, for example that of water or any other suitably chosen molecule.
The scale most often used is that relating to an abundant molecule in all living organisms, namely water.
If A (H2O) denotes the coefficient of attenuation of water, then a relative scale such as the following may be used:Bn=[An−A(H2O)]*1000/A(H2O)
The value of the coefficient of water may be defined as equal to 1 or 0, thereby creating a notation system that is easy to use since water is an essential component of the human body.
Other systems may however be used, according to the way in which the information obtained is expressed (visually). Often a value of 1000 is chosen for bone and a value of −1000 is chosen for air.
The information processing of a sufficient number of cross scannings, defining in fact small elementary cells or zones, enables a set of linear equations to be solved provided that the number of scannings is equal to the number of cells.
The editing and use of the information are carried out by an associated computer.
The computer collects the set of data and then calculates the value of the coefficient of attenuation of each elementary zone.
The information obtained from these calculations is expressed by a map of the tomographic sectional plane.
The set of maps constitutes the three-dimensional scanner image of the analysis, which permits longitudinal or transverse sections.
The medical interpretation is thus based on a real internal image of the tissues.
Such images enable the condition of certain bones, as well as the condition of the brain, to be checked in order to detect a tumour or other anomaly.
The investigations are preceded or completed by other investigations, for example ultrasound echography or magnetic resonance imaging.
Scanning and the methods that it has introduced remain an essential tool of medical investigation.
At the start, a series of angular displacements of the order of 3° were carried out, repeated some hundred times.
The improvements that have been introduced since then enable a plurality of beams to be combined with detection strips of a sufficient length so as to multiply the number of measurements made at any one time thanks to multiple detectors.
In the fifth generation scanners detector strips are used perpendicular to the sectional plane in order to prevent any shift or displacement.
The image that is obtained is the result of a stepwise process:                obtaining values of the attenuations for each projection;        calculations of the values of a profile;        matrix representation of each sectional plane;        conversion of each representation by means of a specific map;        establishment of a spatial cartographic system.        
Nowadays volumes of each elementary zone of the order of mm3 are obtained.
However, this is far from the microscopic scale since the number of living cells is of the order of 1 billion per mm3. Human cells have on average a size of 10 μm. The microorganisms that are found in the human body may have a size of the order of 1 μm3.
The early detection of cancer presupposes a considerable gain in definition. However, the length of time the system is used for a specific patient cannot exceed a certain economic threshold. Above all however, increasing the number of profiles increases the overall radiation dose.
However, it is known that the development of a cancerous nodule accelerates when it causes an associated vascularisation, this phenomenon occurring when a critical size is reached, say for example 50 to 500 microns. In the conventional procedures the radiation dose and the calculation time are multiplied by 8000 in order to achieve the definition equal to 50 μm.